Mysterious ripples discovered in stellar disc

The three rows show images of AU Mic’s disc over three different years. Black central circles show where the light of the central star has been blocked off to reveal the fainter disc. The scale at the top of the picture indicates the diameter of the orbit of the planet Neptune in the Solar System (60 AU). The brightness of the outer parts of the disc has been artificially brightened to reveal the faint structure.

Credit: ESO, NASA & ESA

Astronomers have discovered wave-like features within the dusty disc of a nearby star that have never been seen before.

The structures were seen in the disc of AU Microscopii (AU Mic), a young star located nearby.

Astronomers had been observing AU Mic’s disc for signs of warped features, as these can give away the location of unknown exoplanets. In 2014, they turned to ESO’s newly installed SPHERE instrument on the VLT to facilitate their search and discovered the unusual phenomena.

The team then looked at earlier images of the same disc captured by Hubble in 2010 and 2011 to see if they could spot the ripples. They found that not only were the features visible in the earlier Hubble images, but that they had changed over time, leading to the conclusion that the ripples are moving like those in water.

“Our observations have shown something unexpected. The images from SPHERE show a set of unexplained features in the disc which have an arch-like or wave-like structure, unlike anything that has ever been observed before,” says Anthony Boccaletti, LESIA, France, and lead author of the paper.

“We reprocessed images from the Hubble data and ended up with enough information to track the movement of these strange features over a four-year period,” adds team member Christian Thalmann, ETH Zürich. “By doing this, we found that the arches are racing away from the star at speeds of up to about 40,000 kilometres/hour.”

The team are unclear as to what is causing the ripples around the star, but have been able to rule out numerous possibilities such as the collision of two asteroid-like objects and spiral waves caused by instabilities in the system’s gravity.

The team have also discovered that the ripples further away from the star seem to be moving faster than those closer to it. At least three are moving at speeds so fast it is possible they could be escaping from the gravitational attraction of the star.

These speeds have allowed the team to rule out the possibility that the ripples are conventional disc features of the kind that would usually caused by objects disturbing material while orbiting the star. The research shows there must have been something else involved in the process to speed up the ripples and make them move so quickly.

“One explanation for the strange structure links them to the star’s flares. AU Mic is a star with high flaring activity. It often lets off huge and sudden bursts of energy from on or near its surface,” explains co-author Glenn Schneider of Steward Observatory, USA. “One of these flares could perhaps have triggered something on one of the planets — if there are planets — like a violent stripping of material which could now be propagating through the disc, propelled by the flare’s force.”

The team will now observe the system with SPHERE and other facilities including ALMA in Chile in an attempt to solve the mystery.

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